It is very likely that a significant part of temperature variability in the Northern hemisphere during the past seven centuries prior to 1950 is due to volcanic eruptions and changes in the intensity of solar radiation. However, most of the observed increase in global temperature since then is very likely due to the observed increase in atmospheric greenhouse gas concentrations due to human activities. Human activities now clearly affect other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.
It is likely that these increases in greenhouse gas concentrations alone would have caused more warming than observed, had volcanic and human-induced aerosols not offset some of the warming that would otherwise have taken place.
Current climate models that simulate the observed temperature evolution on each of six continents provide stronger evidence of human influence on climate than was available in the 2001 Third Assessment Report (TAR). Difficulties remain in simulating temperature changes at smaller scales, where natural climate variability is larger and makes it harder to estimate the current and future impact of greenhouse gas increases due to human activities.
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